A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Current methods to evaluate printability of structures processed by a lithographic process that uses formation of a periodic image on a substrate may be based on the technique known as CDSEM (Critical Dimension Scanning Electron Microscopy). This is a technique in which, for example, knowing the expected dimensions of a structure, for example a photoresist grating, a dedicated Scanning Electron Microscope (SEM) is used to capture images of the grating with nanometric resolution and an image processing algorithm detects the edges of the grating lines to evaluate their Critical Dimension (CD). The behavior of CD with respect to printing conditions (namely focus and dose) can yield the edges and centre of the process window of the lithographic process.
Alternative methods can be used based in scatterometry techniques. One example of them requires the use of an angularly resolved scatterometer to reconstruct the photoresist cross-section profile based on the light scattered by the grating (CD reconstruction). This technique can provide more information than the CDSEM, but it requires additional previous knowledge of the grating characteristics and material properties.
Such methods to evaluate printability in semiconductor lithography of structures processed by a lithographic process that uses formation of a periodic image on a substrate are time-consuming and/or require a great deal of knowledge of the printed structures and underlying material stack properties.
In the case of CDSEM, it is required to dedicate a costly tool for an extended period of time in order to take images of each required target at each required point in a Focus Energy Matrix (FEM) wafer in order to build a map of the CD as function of the focus-dose conditions. These measurements are done at independent lines in the grating, for which they are highly noise sensitive.
In the case of using scatterometry tools for CD reconstruction (such as angularly resolved scatterometers) an important knowledge of the targets to be measured is needed: CD and pitch ranges, material properties, line roughness, etc. In order to take all these parameters into consideration, a process of CD recipe creation is needed, which typically takes 8 to 40 man-hours and requires additional thin-film and CDSEM measurements.
Currently, for lithography process monitoring products, scatterometry methods are preferred providing much better throughput performance comparing to CDSEM. However, CDSEM is still used during the setup phase, especially for determination of edges and centre of the process window of the lithographic process. The mixed use of two different such metrology approaches adds extra complexity and requires longer setup lead time.